1. Field of the Invention
This invention relates to a method for data communications, an on-board communication equipment and a road side communication equipment, which use synchronizing signals as communication signals for sending communication data and process subsequent communication data according to the synchronizing signals included in received communication signals.
2. Description of Related Art
In a communication system, for example, a transmitter and a receiver are synchronized to send and receive signals by adding a synchronizing signal to the start of each transmission signal. In this case, therefore, the data length of the synchronizing signal must be set as long as possible to prevent influences of external noises and ensure communications in wireless communications. Setting a longer data length for communications, however, causes the communication time to be extended according to the surplus data length. This also becomes a disadvantage for communications when the communication time is limited as described below.
Such a disadvantage occurs in communications between an on-board communication equipment mounted in a vehicle such as a car and a road side communication equipment provided in a communication area on a road when the vehicle passes the communication area. For example, there is an automatic system for data communication, wherein each vehicle is provided with an on-board communication equipment and the road (eg., a tollgate on an expressway) is provided with an antenna so that the vehicle can pay the traffic fee to be decided by traffic section automatically through a wireless communication processing executed when the vehicle passes under the road side communication equipment. Construction of such a communication system expects advantages of reducing the personnel expenses for collecting traffic fee, the number of processes for the work, as well as to eliminate traffic jams at the tollgate, since vehicles are not required to stop at the tollgate any longer. Under the above circumstance, therefore, a communication processing for collecting traffic fee must be completed assuredly within a short time while the on-board equipment of the object vehicle passes the communication area of the antenna.
In order to solve the above problems and complete the communication processing assuredly, there is also a communication system which uses a plurality of synchronizing signals, each of which has a data length different from each other. In this instance, the following configuration shown in FIG. 16 may be considered for identifying synchronizing signals. In this configuration, a first synchronizing signal UW1 (Unique Word 1) of 32 bits (4 octets) and a second synchronizing signal UW2 (Unique Word 2) of 16 bits (2 octets) are set as defined below as two types of synchronizing signals having different data lengths from each other.
UW1 0111 1100 1101 0010 0001 0101 1101 1000 UW2 1001 0010 1000 0111
A shift register 1 is used to hold 32-bit data. When receiving digital signals as received data, the signals are shifted sequentially in the register 1. Each bit data in the shift register 1 is connected to each input terminal of the comparator 2. The comparator 2 outputs a detection signal when the input data has the same bit pattern as that of the synchronizing signal UW1. A 16-bit comparator 3 is connected so that the input terminal receives the higher 16-bit data from the shift register 1. The comparator 3 outputs a detection signal when the input data has the same bit pattern as that of the synchronizing signal UW2.
When starting a communication, the UW1 having a longer data length is added to the signal indicating the start of the communication from a transmitter (not illustrated) before communication data is sent. This is to synchronize signals assuredly to send and receive data. The receiver, when receiving this start signal, enters received data that are demodulated to digital signals to the shift register 1. Then, when the first comparator 2 detects the synchronizing signal UW1 in the received data, the receiver starts another new communication according to the synchronizing signal UW1.
When a synchronized communication is started thus, the transmitter, when sending subsequent data, adds the synchronizing signal UW2 to the start of the communication signal to shorten the synchronizing time and sends subsequent signals and communication data continuously. Receiving the transmission signal, the receiver continues the communication according to the synchronizing signal UW2 if the synchronizing signal UW2 is included in the received data.
Thus, the synchronizing signal of the longer data length (32-bit) UW1 is used for determining the timing for the entire communication when the communication is started and the synchronizing signal of the shorter data length (16-bit) UW2 is used for determining timing of subsequent signals, so that the communication can be synchronized to send and receive data assuredly within a limited communication time, and accordingly the communication efficiency can be improved significantly.
When constructing a circuit for synchronizing the communication as described above, two comparators 2 and 3 are needed to detect each synchronizing signal UW1 and UW2 so as to identify each of synchronizing signals having different data lengths from each other. As a result, the number of bits necessary for forming such comparators is increased and accordingly, the following technical problems still remains for integrating those into a semiconductor integrated circuit; the circuit must be more simplified and reduced in size for mounting.